NimBLE Security

The Bluetooth Low Energy security model includes five distinct security concepts as listed below. For detailed specifications, see BLUETOOTH SPECIFICATION Version 4.2 [Vol 1, Part A].

  • Pairing: The process for creating one or more shared secret keys. In LE a single link key is generated by combining contributions from each device into a link key used during pairing.
  • Bonding: The act of storing the keys created during pairing for use in subsequent connections in order to form a trusted device pair.
  • Device authentication: Verification that the two devices have the same keys (verify device identity)
  • Encryption: Keeps message confidential. Encryption in Bluetooth LE uses AES-CCM cryptography and is performed in the Controller.
  • Message integrity: Protects against message forgeries.

Bluetooth LE uses four association models depending on the I/O capabilities of the devices.

  • Just Works: designed for scenarios where at least one of the devices does not have a display capable of displaying a six digit number nor does it have a keyboard capable of entering six decimal digits.
  • Numeric Comparison: designed for scenarios where both devices are capable of displaying a six digit number and both are capable of having the user enter “yes” or “no”. A good example of this model is the cell phone / PC scenario.
  • Out of Band: designed for scenarios where an Out of Band mechanism is used to both discover the devices as well as to exchange or transfer cryptographic numbers used in the pairing process.
  • Passkey Entry: designed for the scenario where one device has input capability but does not have the capability to display six digits and the other device has output capabilities. A good example of this model is the PC and keyboard scenario.

Key Generation

Key generation for all purposes in Bluetooth LE is performed by the Host on each LE device independent of any other LE device.

Privacy Feature

Bluetooth LE supports an optional feature during connection mode and connection procedures that reduces the ability to track a LE device over a period of time by changing the Bluetooth device address on a frequent basis.

There are two variants of the privacy feature.

  • In the first variant, private addresses are resolved and generated by the Host.
  • In the second variant, private addresses are resolved and generated by the Controller without involving the Host after the Host provides the Controller device identity information. The Host may provide the Controller with a complete resolving list or a subset of the resolving list. Device filtering becomes possible in the second variant when address resolution is performed in the Controller because the peer’s device identity address can be resolved prior to checking whether it is in the white list.

Note: When address resolution is performed exclusively in the Host, a device may experience increased power consumption because device filtering must be disabled. For more details on the privacy feature, refer to BLUETOOTH SPECIFICATION Version 4.2 [Vol 3, Part C] (Published 02 December 2014), Page 592.